Polycarbonates are themselves self-extinguishing materials. However, for certain purposes, such as building, domestic appliances and the aeronautical industry, a higher level of fire resistance is required. Previous polycarbonate fireproofing agents have included halo compounds, such as decabromodiphenylether, used as additives, or tetrachloro- or tetrabromobisphenol A, used as polycondensation agents. The drawback of such products is that the resulting materials are liable to release poisonous, corrosive hydrohalic acids (hydrochloric or hydrobromic acid) during combustion.
Phosphorus derivatives are commonly used to increase the fire resistance of many polymers. Such products are neither toxic nor corrosive during combustion, and they possess combustion-inhibiting properties, believed to result from the formation of carbon residues and non-combustible materials, which slow down the spread of combustible gas to the combustion zone.
U.S. Pat. No. 4,036,809, for example, suggests the use of xylenephosphonic acid diesters, with halogenated nuclei, as additives to fireproof various polymers, such as polyesters, polystyrene, polyurethanes, polyolefins, and even polycarbonates. The quantity of such diesters added to the polymer depends on the type of polymer, and ranges from a few percent to a few tenths of the total. The need for such large quantities to fireproof the polymer adequately makes their use extremely expensive for industrial purposes. Furthermore, the presence of large quantities of additives can detract from the mechanical properties of the fireproofed polymer, and affect behaviour during their transformation into shaped products.
Alkaline metal salts are also known to be effective fireproofing agents, by acting as radical-recombination catalysts, and thereby as flame inhibitors.
In French patent application No. 79 02836 of Feb. 5, 1979 (publication No. 2 447 940), in the present assignee's name, it was shown that inclusion of phosphonic hemiester groups in the chains of a polycarbonate, produced by reaction between bisphenol A and phosgene, with 0.5 to 2 molar percent bisphenol A replaced by an alkaline hemiester of bis(hydroxy-4 phenyl) alkylphosphonic acid, provided a fire-resistant polycarbonate. However, the inclusion of even such small amounts of phosphonic hemiester compound in the polycarbonate chains, by means of such polycondensation, can disturb the kinetics of the polycondensation reaction, and cause difficulties in use, particularly on an industrial scale, in order to obtain a product with the required properties, for instance as regards molecular weights.